An oxide semiconductor film composed of several metal composite oxides has a high degree of carrier mobility and visible ray transmittance. Therefore, it is used in a variety of fields including a switching element, a driving circuit element or the like for a liquid crystal display, a thin film electroluminescence display, an electrophoresis display, a moving particle display.
Of these oxide semiconductor films composed of metal composite oxides, an oxide semiconductor film composed of indium oxide-gallium oxide-zinc oxide, which is called IGZO, is used most widely. In addition to IGZO, indium oxide-zinc oxide (IZO), one obtained by adding zinc oxide to tin oxide (ZTO), one obtained by adding gallium oxide to indium oxide-zinc oxide-tin oxide or the like are known. They differ from each other in easiness in production, cost, properties or the like, and hence, they are used appropriately according to applications.
Of these, an oxide semiconductor film composed of an oxide of In, Ga and Zn (IGZO) or composed mainly of these oxides has attracted attention in each application since it has an advantage that the carrier mobility is large as compared with an amorphous silicon film (see Patent Documents 1 to 7, for example).
Generally, a sputtering target used for forming an oxide semiconductor film composed of indium oxide-gallium oxide-zinc oxide is produced by steps of mixing of raw material particles, pre-firing, pulverizing, granulating, molding, sintering and reducing. Due to the large number of steps, this target has disadvantages of low productivity and high cost. In addition, although the bulk resistance of a target is decreased by reduction, the conductivity thereof after reduction is utmost 90 S/cm (specific bulk resistance: 0.011 Ωcm). That is, it is impossible to obtain a target with a sufficiently low resistance.
Therefore, it is desirable to omit even one of the above-mentioned steps. However, no improvement has been made on the steps, and targets are currently produced by conventional production steps.
Meanwhile, it is known that an IGZO sputtering target is composed mainly of a compound shown by InGaO3(ZnO)m (m is an integer of 1 to 20). Further, a compound shown by InGaO3(ZnO)2. InGaO3(ZnO)3, InGaO3(ZnO)4, InGaO3(ZnO)5 or InGaO3(ZnO)7, and methods for producing thereof are known.
However, as for the raw material powder used in these methods, description is only made that a particularly preferable particle size is 10 μm or less. In addition, while generation of each compound is confirmed, no description is made on the specific bulk resistance value. Accordingly, these compounds are defective to be used in a sputtering target.
In addition, a problem occurs that, when sputtering (DC sputtering, for example) is conducted using an IGZO sputtering target, a compound shown by InGaO3(ZnO)m is grown extraordinary to cause abnormal discharge, resulting in the formation of a defective film.
Further, the resulting oxide film has poor resistance to chemicals, and hence, is dissolved in a PAN (phosphoric acid-acetic acid-nitric acid)-based etching solution which is used in etching for metal wires. As a result, when producing a thin film transistor or the like using a semiconductor film, restrictions are imposed on the structure or the process.    Patent Document 1: JP-A-2006-165527    Patent Document 2: JP-A-2006-165528    Patent Document 3: JP-A-2006-165529    Patent Document 4: JP-A-2006-165530    Patent Document 5: JP-A-2006-165531    Patent Document 6: JP-A-2006-165532    Patent Document 7: JP-A-2006-173580
An object of the invention is to provide a target for physical film formation which can suppress occurrence of abnormal discharge during sputtering, and to provide an oxide semiconductor film having a high PAN resistance.